1. Field of the Invention
The invention relates to a process for the production of an anode for electrolytic capacitors as well as to anodes produced by the process.
2. Description of the Related Art
For a capacitor comprising two metal plates separated by means of a dielectric of a given strength, the capacitance is a function of the dielectric constant of the dielectric, the vacuum dielectricity, the geometric area, as well as the distance between the metal plates. In order to arrive at higher capacitance values at a given area, the thickness of the dielectric can be decreased, the effective surface of the metal plates can be increased by roughing and a dielectric with a higher dielectric constant can be used.
It is known that, compared with other metal oxides, aluminum oxide has considerable advantages due to the fact that from an aluminum foil can be generated anodically with electrochemical etching, films with the formation of aluminum oxide with good insulating properties and with high roughness such that very high capacitance values can be attained in spite of the relatively low dielectric constant of aluminum oxide in comparison with other, metal oxides, such as those of titanium or tantalum.
GB-B 2 056 503 discloses a process for the production of an electrolytic capacitor in which a flexible substrate is used with a surface onto which an anodizable metal is vapor-deposited. The vapor deposition takes place at an angle of incidence of less than 60.degree. C. in the presence of oxygen and at a partial pressure not greater than 10-.sup.4 Torr to form a porous metal layer. The porous metal layer is subsequently anodized and a capacitor is wound from such foils. The substrate can be aluminum and the anodizable metal can be tantalum or an alloy of two or several metals.